SLC12A1 and colitis: Ccc3 and Ccc2 appear to act epistatically with Ccc1. Globally, the additive effects of Ccc1, Ccc2 and Ccc3 and the Ccc1 × Ccc3 and Ccc2 × Ccc3 interactions explain 26.4 % of the observed variance in colitis score.